1. Field of the Invention
The present invention relates to an inverter control apparatus for driving a three-phase induction motor, and more particularly, to an inverter control apparatus which changes between two-phase-modulation, in which peak values of three-phase-voltages of an inverter are fixed, and three-phase-modulation.
2. Description of the Related Art
U.S. Pat. No. 4,641,075 proposes a PWM (pulse-width modulation) control apparatus (hereinafter referred to as the inverter control apparatus) for a three-phase inverter, in which a two-phase-modulation is adopted to reduce electromagnetic-wave noise and switching-power-loss and to improve the power-conversion-efficiency. The two-phase-modulation fixes one phase-voltage out of three phase voltage of an inverter to a certain level in sequence and modulates other two phase voltages.
U.S. Pat. No. 4,847,743 proposes a device to change over from the above two-phase-modulation to the three-phase-modulation or vice-versa. The two-phase-modulation is changed over to the three-phase-modulation when a voltage-vector-command-signal (which corresponds to an amplitude of the inverter-output-voltage) becomes less than a predetermined value. In the three-phase-modulation of the inverter control apparatus, the PWM-control is carried at according to a difference between a set-current-signal and detected or real motor-current (e.g., PI (proportional and integrated) processing of deviation) to provide three-phase alternating voltages, which is applied to a three-phase induction motor. The two-phase-modulation fixes each phase-voltage of the inverter to a certain value at a certain phase-angle of the set-current-signal (sinusoidal wave).
The above mentioned two-phase-modulation is effective to reduce the magnetic noise and switching power loss, and also improve the efficiency of the power conversion. However, it has the following problems.
That is, the conventional two-phase-modulation fixes each phase-voltage during a certain phase-angle range of the set-current-signal, which is a sinusoidal wave signal, to a fixed voltage and controls other unfixed phase-voltages to voltages corresponding to the set-current-signal. To put it more concretely, the current wave, which has an angle range of 360.degree. in a cycle, of the set-current-signal of each phase is divided into 60.degree. phase-intervals so as to include an interval having phase-angles 30.degree. on the left and right from the center of the peak value of the current wave, and the peak value of the set-current-signal in each phase-interval in the positive or the negative direction is fixed as a peak value in the positive or the negative direction, and the rest of the set-current-signals are not fixed but controlled through a minor loop of a motor current feedback circuit.
However, in this case, the current of the set-current-signal of the phase voltage to be fixed increases and the currents of other unfixed phases are controlled, irrespective of the current increase, through a minor current-feedback loop according to a sinusoidal set-current-signal. Accordingly, the fixed phase is caused as if a disturbance is input thereto, thereby changing the set-current-signal more abruptly than that in the three-phase-modulation to distort the motor-current wave.
Japanese Unexamined Patent Publication Hei 6-233549 filed by the same applicant as this application, on the other hand, solves the above problem. In this reference, a deviation between set-current-signal of each phase and a real-current-signal of the corresponding phase is processed through a PI control circuit, which is compared to a triangular wave of a certain frequency by a comparator to generate a PWM signal, which drives an inverter to control the phase voltages applied to a motor. In order to eliminate the distortion of the motor-current, an offset value that is necessary to fix a phase voltage is added to or subtracted from the triangular wave corresponding to the phase to be fixed and also a certain offset value is added to or subtracted from the triangular waves corresponding to other unfixed phases.
However, since the motor-current is feedback-controlled according to a set-current-signal and a phase voltage to be applied to the motor is determined according to a deviation between the set-current-signal and the real motor-current, there is a phase difference between the set-current-signal and the phase voltage to be applied to the same phase. As a result, if the above fixing is carried at a phase angle where the absolute value of the set-current-signal becomes a peak value, the phase angle is fixed where the absolute value of the phase voltage is not its peak, and since the offset value, which is added or subtracted to or from the fixed phase, is added to or subtracted from the unfixed phases so that the modulation percentage exceeds 100%, the phase voltage to be applied to the motor and motor-current is distorted.
The cause and effect of the distortion of the phase voltage wave in the motor operation is described next.
The three-phase induction motor operates as a ordinary electric motor which is referred to as the power running condition and also operates as a generator which is referred to as the regeneration" running condition. FIG. 14 is a vector diagram in which the phase voltage vector and the phase current vectors are plotted in the d-q coordinates, and FIG. 15 is a vector diagram showing the relationship between the above vectors in the "regeneration running condition.
In FIG. 14 and FIG. 15, a reference character .PHI.0 indicates an exciting-magnetic-flux-vector component of the rotating-field-magnetic-flux-vector, extending in the same direction as the d axis. A reference character E0 is a voltage vector induced in the motor coil by the exciting magnetic-flux-vector .PHI.0 ahead of the exciting magnetic-flux-vector by a phase angle 90.degree.. A reference character I indicates a current vector of the motor coil, Id indicates an exciting-current-vector component of the current vector I, Iq indicates a torque-current component of the current vector, R indicates a motor coil resistance, L indicates a motor coil inductance, and RI is a voltage drop across a resistor having a resistance R when the current I flows therethrough in the same phase as the current vector I. RId is a d-axis component of the voltage drop vector RI, RIq is a q-axis component of the voltage drop vector RI, .omega. is an angular velocity of the current vector I, .omega.LI is a voltage drop vector across the motor coil having an inductance L, and .omega.LIq is a q-axis component of the voltage drop vector .omega.LI and is generated when the q-axis component of the current vector I flows through the motor coil having its inductance L at a phase angle 90.degree. ahead of the torque current component Iq. .omega.LId is a d-axis component of the voltage drop component .omega.LI and is generated when the d-axis component and the exciting current Id of the current vector I flows through the motor coil having its inductance L at a phase angle 90.degree. ahead of the exciting current Id. Reference character V indicates a voltage vector applied to the motor coil, which is a resultant of a voltage vector E0, the voltage drop vector R1 and the voltage drop vector .omega.LI.
In the power running condition shown in FIG. 14, the voltage vector V advances by a phase angle .theta. ahead of the current vector I. However, in the regeneration running condition shown in FIG. 15, since the torque generated in the motor becomes negative and the torque current component Iq points the negative direction of the q axis, the advancing phase angle .theta. of the voltage vector V ahead of the current vector I becomes significant. The current vector I at this moment is controlled to be equal to the set-current-signal by a current-feedback-control loop so that the advancing phase angle of the voltage vector V relative to the set-current-signal becomes equal to the angle .theta.. As a result, the phase difference in the regeneration running condition becomes greater than that in the power running condition in the above described fixing mode, thereby distorting of the phase voltage in the regeneration running condition much more than in the power running condition.
As described above, the three-phase-modulation mode is more desirable during the regeneration running condition of the three-phase induction motor, and the two-phase-modulation is more desirable during the power running condition thereof.
However, since the changeover between the three-phase-modulation and the two-phase-modulation is carried according to the set-current-signal in the above described conventional apparatus, when the regeneration running condition is changed over to the power running condition, the real-current-signal or the detected current retards relative to the phase voltage corresponding to the set-current-signal and the detected current becomes a value for the changeover from the regeneration running condition to the power running condition a certain time after the real changeover. As a result, if the changeover from the regeneration running condition to the power running condition is detected from the change of the set-current-signal, in order to change the three-phase-modulation mode to the two-phase-modulation mode, the two-phase-modulation is still carried still in the regeneration running condition, resulting in significant distortion of the phase voltage, thereby distorting the motor-current and causing vibration of the motor.